Power-flattened seed-blanket reactor core

ABSTRACT

A seed-blanket breeder reactor core having a flattened power density distribution resulting from selective loading of fissile material in the reactor core to achieve a desired radial reactivity distribution. The seed geometry of fuel modules near the periphery of the core is altered so that their reactivity worth is greater than centrally located fuel modules. The increased reactivity of the peripheral modules causes the gross power distribution in the core to flatten.

ii 1 States ate E [151 3,640,844 Shank et al. Feb. 8, 1972 [54]POWER-FLATTENED SEED-BLANKET 3,351,532 11/1967 Raab, Jr. et a] ..176/17REACTOR CORE 3,338,790 8/1967 Ackroyd et al. ..l76/l8 3,432,389 3/1969Stern ..l76/40 [721 Rchard Shank 3,15s,543 11/1964 Sherman et al...l76/17 luck"; David 3,252,867 5/1966 Conley ..l76/l8 Jr., all ofPlttsburgh; Robert T. Bayard, 3 341420 9/1967 Sev 176/18 Bethel Park aof Pa y [73] Assignee: The United States of America as PrimaryExaminer-Carl D. Quarforth p f f y the Us Atomic gy AssistantExaminer-Harvey E. Behrend Commission Attorney-Roland A. Anderson 221Filed: Nov. 7, 1969 [57] ABSTRACT 21] Appl. No.: 874,735

A seed-blanket breeder reactor core having a flattened power densitydistribution resulting from selective loading of fissile material in thereactor core to achieve a desired radial rem} c tivity distribution. Theseed geometry of fuel modules near the [58] Field of Search ..l76/l7,18,40 periphery ofthe core is altered so that their reactivity worth is[56] References Cited greater than centrally located fuel modules. Theincreased reactivity of the peripheral modules causes the gross powerUNITED STATES PATENTS distribution in the core to flatten.

3,335,060 8/1967 Diener ..176/l7 X 4 Claims, 6 Drawing Figures PATENTEUma ma SHEET 1 BF 5 PRIOR ART IN VEN TORS. Richard C. Shank Carl E. ZuckerDavid H. Jones Harry E Raab, Jr. Robert 7? Bayard M 4- AT TORNE Y.

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SHEET 2 0F 5 INVENTORS. Richard C. Shank E. Zucker "d H. Jones Harry I?Raab, Jr. BY Robert I Bayard 4% ATTORNEK SHEET 3 0F 5 INVENTORS. RichardC. Shank Carl E. Zuc David H. J0

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nes Harry E Raab, Jr. BY Robert TBayard 4-* ATTORNEY.

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David H. Jon Harry E R r. BY Robert 7? rd INVEN Richard C. Shan Carl E.Zucker ATTORNEY.

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SHIEET 5 BF 5 2s 2e 2e INVENTORS. Richard C. Slgank Fig. 5 Carl E.Zucker David H. Jones Harry I-.' Raab, Jr. BY Robert T Bayard ATTORNEY.

POWER-FLATI'ENED SEED-BLANKET REACTOR CORE BACKGROUND OF THE INVENTIONThe invention described herein was made in the course of, or under,Contract AT-l l-l-GEN-l4 with the United States Atomic EnergyCommission.

This invention relates to flattening the power density distribution overa nuclear reactor core and more particularly to flattening the powerdensity distribution over the core of a seed-blanket-type breederreactor by altering the seed geometry of the peripheral fuel modules.

The following terms will be used in describing the invention:

Fissile materialThe term will be used to designate material whichfissions upon the absorption of a thermal neutron.

Fertile materialThe term will be used to designate material whichtransforms into fissile material upon the capture of neutrons.

Neutron multiplication, K,The term will be used to designate the ratioof the number of neutrons present at a given time to the number presentone generation earlier.

FuelThe term will be used to designate either fissile or fertilematerial or a combination of both, within the reactor core.

Seed regionThe term will be used to designate a region formed by nuclearfuel wherein the primary activity is the fissioning of fissile material;

Blanket regionThe term will be used to designate a region formed bynuclear fuel wherein the primary activity is the transforming of fertilematerial to fissile material.

Neutron fluxThe term will be used to designate the number of neutronspassing through a unit area per unit time.

Power densityThe term will be used to designate the power generation perunit volume of a nuclear reactor core.

Seed geometry-The term will be used to designate the physicalconfiguration of fuel elements in the seed region.

Blanket geometryThe term will be used to designate the physicalconfiguration of the fuel elements in the blanket regron.

A breeder reactor increases the fissile material inventory over corelifetime while concurrently producing power. When fission occurs,neutrons and energy are emitted. The fissile material inventory in abreeder reactor is increased when fertile material such as thorium-232absorbs neutrons emitted during the fission process. When an atom ofthiorium-232 captures a neutron, it becomes thorium-233 whichradioactively decays emitting beta particles to protactinium-233.Protactinium-233 radioactively decays emitting beta particles touranium-233, a fissile material. The chief source of neutrons forabsorption by the fertile material is the fissioning of fissile fuel. Aminor source of neutrons is the fast fissioning of nonfissile materialsuch as thorium-232 and protactinium-233. Of the net number of neutronsproduced for each atom of fissile material destroyed, one neutron mustbe subsequently absorbed in the fissile material to sustain the chainreaction and keep the reactor critical. Concurrently, the fertilematerial must absorb at least one additional neutron to replace thefissile atom destroyed if breeding is to be accomplished. It is therefornecessary that the ratio of neutron production from fissions per neutronabsorption in fissile material be greater than 2.0 if breeding is totake place. Furthermore, it is necessary for this ratio to besufficiently greater than 2.0 to account for neutrons lost throughparasitic capture (the absorption of a neutron that does not result in afission) by moderator, structure, fission products and impurities in thefuel. Uranium-233, cooled and moderated by light water, is a materialwherein the ratio of neutron production per thermal neutron absorptionis greater than 2.0.

Initially, the seed region in a seed-blanket breeder reactor containssufficient quantities of fissile material to insure that the primaryactivity in the region is the fissioning of the fissile material. Priorart seed-blanket breeder reactors have provided self-sustained breedingwith a seed region fueled with uranium-233 and a blanket region fueledwith thorium-232.

The tho'rium-232-uranium-233 fuel element arrays were contained inseed-blanket modular fuel units. Several of these modules in anassembled array comprised the reactor core. Often, the several modularunits that comprised the core were surrounded by a natural thoriareflector blanket. U.S. Pat. No. 3,351,532 to H. F. Raab, Jr. et al.typifies this approach. Also, breeder reactors have providedself-sustaining breeding when the blanket region was fueled with somefissile U-233 in addition to the thorium.

An important consideration in nuclear reactor design is the powerdensity distribution in the core throughout core lifetime. A reactorcore fueled with a plurality of identical fuel modules, i.e., fuelmodules of identical size and shape having the same fissile-fuelcomposition, would be initially characterized by an overall flux andpower shape which could be approximated by J, where J, represents over alarge portion of the core the distribution of the zero order Besselfunction of the first kind. Thus, the highest power density would tendto occur at the center of the core and decrease radially to a minimum atthe core periphery. Further, this distribution would generally changeover the lifetime of the core. It is desirable to the variations inpower density throughout the core and to obtain an even sharing of poweramong the modules throughout core lifetime. For the power distributionacross the core to be as uniform as possible the ratio of maximum poweroutput to average power output should be close to unity. Balancing thepower among the modules in the core is known as radial power-flattening.A flat power distribution that is maintained throughout core lifetime isdesirable because it results in a more even bumup of the nuclear fuel,and hence, a more uniform burden on the fuel elements in the core. Also,when the power distribution in the core is flat, the flow required tocool the reactor can be more uniform. And finally, a more uniform powerdistribution results in lower power costs because the core size for agiven power output is minimized and the average energy output of thefuel elements is maximized without exceeding maximum fuel depletionlimits.

A flat power distribution is also important in the core of a breederreactor, such as a light water breeder reactor, since neutronabsorptions by protactinium-233 and xenon-l35 reduce the breedingperformance of the core. Unlike other parasitic absorbers, the effect ofprotactinium-233 and xenonon conversion ratio (the number of fissionableatoms produced per fissionable atom destroyed in a breeder nuclearreactor) is a function of neutron flux level and neutron flux isproportional to power density. The power rating of the core consistentwith good breeding performance can be enhanced by power flattening ifthis is accomplished by flux flattening since neutron losses toprotactinium-233 and xenon-135 are minimized.

Prior art approaches to power density flattening have included radiallyvarying the fissile fuel composition from the center of the core to thecore periphery. Fuel of lower enrichment (number of fissile atomsavailable per unit volume) is used in the center of the core where thepower density would ordinarily be highest. The fuel enrichment radiallyincreases toward the core periphery where the power density wouldordinarily be lowest. This raises the problem of increasing the numberof fuel elements having specially tailored loadings. A further problemwith conventional radial fuel zoning is that large amounts ofuranium-233 must be added to the reactor core in order to achieve arelatively flat power density distribution. The excessive amounts ofuranium-233 that must be expended makes conventional radial fuel zoninginefficient. Another technique, described in US. Pat. No. 3,341,426 toC. P. Gratton et al., is to use neutron absorbers in those regions ofthe core where peaks in the power density distribution occur. But thisapproach is deleterious to breeding since it involves a deliberate lossof neutrons. Another prior art method, described in US. Pat. No.3,267,00l to P. Greebler, arranges the neutron reflector material,fissile fuel material and blanket fuel material into specific zones totake advantage of the properties of the materials to provide a flatpower density distribution over the fissile fuel portion of the core.

The seed-blanket breeder reactor described in US. Pat. No. 3,351,532 toH. F. Raab, Jr. et al. and US. Pat. No. 3,335,060 to R. L. Diener,provides for control of the reactivity level of the core by movement ofa portion of the nuclear fuel in each fuel module. The movable fuelconcept has potential for flattening the power density distribution inthe core by moving the peripheral fuel modules to a higher, morereactive position than the centrally located fuel modules. It has beendemonstrated by analysis of a seed-blanket breeder reactor fueled withuranium-233 and thorium-232 and 15 feet in diameter and 8 feet in heightthat a relatively flat power density distribution could be achieved byvarying the positions of the movable fuel in the inner and outer fuelmodules in the range 0.5 to 1.0 feet. This amount of misalignment is notoptimum from the standpoint of breeding performance, reactivity worth,and core lifetime. This will be described in more detail below. Theresults of examining power flattening in a smaller core (8-footdiameter) for a seed-blanket-type breeder reactor indicated thatselective positioning of the movable fuel was even less desirable thanin the above-mentioned larger core. The difference in the movable fuelpositions of the outer and centrally located fuel modules was in therange 1.5 to 2.5 feet. This difference in movable fuel positions hasseveral disadvantages. The large difierence in fuel positions results inincreased neutron leakage from the core, thereby adversely affectingbreeding performance. Further, an axially skewed power densitydistribution may result, thereby limiting thermal performance of thereactor. Also, as the core depletes, any unpredicted changes in thegross power distribution would have to be controlled by furtheradjustment of the movable fuel positions. And further, a largedifference in movable fuel position at the beginning of life does notpermit sufficient flexibility for control throughout core lifetime.Thus, it can be seen that movable fuel misalignment to achieve powerflattening is undesirable because of the loss in reactor performance.

Accordingly, it is an object of the invention described herein toprovide more effective flattening of the power density distribution inthe core of a seed-blanket reactor with movabie fuel control than can beachieved by movable fuel misalignment.

It is a further object of this invention to provide a more efficientpower flattening than is achieved by conventional radial fuel zoning.

It is a further object of this invention to provide power flatteningthat results in a minimal leakage of neutrons from the core.

SUMMARY OF THE INVENTION The invention described herein provides aseed-blanket breeder reactor core cooled and moderated by light water,surrounded by a reflector blanket and comprimd of a plurality of fuelmodules each having a movable assembly including a central seed regionand a stationary assembly including an annular seed region and anannular blanket region, each of said fuel modules comprised of fuelelements with the same enrichment, the improvement wmprising fuelmodules around the periphery of the core having a greater reactivityworth than the centrally located fuel modules. The annular seed regionthickness of the peripheral fuel modules is greater than the annularseed region thickness of the centrally located fuel modules therebyreducing local power gradients across the peripheral fuel modules andflattening the power density distribution.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more clearlyunderstood from the following description and accompanying drawingswherein:

FIG. 11 is a horizontal sectional view through a l2-rnodule seed-blanketbreeder reactor core of the prior art employing no power-flatteningtechnique;

FIG. 2 is a horizontal sectional view through a l2-module seed-blanketbreeder reactor core illustrating the increased annular seed regionthickness of the peripheral fuel modules;

FIG. 3 is a horizontal sectional view through a l2-rnodule seed-blanketbreeder reactor core illustrating a uranium-bearing blanket encirclingthe peripheral fuel modules;

HO. 4 is a horizontal sectional view through a l2-module seed-blanketbreeder reactor core illustrating a uranium-bearing blanket encirclingthe peripheral fuel modules wherein the peripheral fuel modules have anincreased annular seed region thickness;

FIG. 5a is a horizontal axial section through a typical seedblanket fuelmodule; and

H6. 5 is a vertical projection of FIG. 5a.

DESCRIPTION OF THE PREFERRED EMBODIMENT There are several different coredesigns which can be used for a light water breeder reactor core. Todemonstrate the effectiveness of the invention described herein al2-module core, 8 feet in diameter, has been chosen which employs arelatively widely spaced uranium-233 oxide bearing thoria blanket. Theloose array of blanket fuel rods forming the blanket region of the corelessens hydraulic resistance to the coolant thereby providing a wetblanket region. FIG. 1 illustrates the lZ-module breeder reactor coreused to demonstrate the effectiveness of this invention. Reactor core 10is cooled and moderated by light water and surrounded by reflectorblanket l2. Reflector blanket 112 is composed of thoria-fueled rods (notshown) and captures neutrons which leak from the inner portion of core10. The thickness of reflector blanket 12 is chosen so that the netradial leakage of neutrons from the core is consistent with thecriterion of breeding in the reactor core. Each of the 12 hexagonallyshaped fuel modules 14 is located in one of three regions in core 10.The three central fuel modules with no sides exposed to reflectorblanket 12 are located in region I. The three fuel modules located alongthe periphery of core 10 with two sides exposed to reflector blanket 112are in region II. The remaining six modules located around the peripheryof core 10 with three sides exposed to reflector blanket 12 are locatedin region Ill. The reason for indicating the location of each of thefuel modules 14 as being in either regions I, II, or III will bediscussed below. Furthermore, fuel modules located in region I will bedesignated as centrally located and fuel modules located in regions IIand [[I will be designated as peripheral fuel modules. As illustrated inFIGS. 5 and 5a, each of the fuel modules 14 includes a central movableassembly 22 and an annular stationary assembly 24 surrounding movableassembly 22. Movable assembly 22 includes central blanket region 26 andcentral seed region 28, lllustrated in FIGS. 1, 5, and 5a. Stationaryassembly 24 includes annular seed region 30 and annular blanket region32. Reactivity control in core 10 is achieved by vertical movement ofmovable assembly 22 within stationary assembly 24. An upward movement ofmovable assembly 22 will result in an increase in reactivity. Seedregions 28 and 30 are fueled with rods containing a mixture ofuranium-233 oxide and thoria. As illustrated in FIG. 5, seed regions 28and 30 are axially zoned, i.e., the fissile-fuel composition is axiallyvaried in zones a, b, and c. Through axial zoning of seed regions 28 and30 it is possible to achieve high reactivity variation and therefore ahigh degree of reactivity control. The metal-to-water ratio in core isapproximately 2.5. The blanket fuel rods (not shown) contain a mixtureof uranium-233 oxide and thoria with the uranium-233 oxide comprising0.5-1.0 weight percent in the preferred embodiment. As previouslymentioned, the blanket rods are arranged in a relatively widely spacedarray. Reflector blanket l2 surrounding the fuel modules is alsocomprised of widely spaced blanket rods.

The invention described herein relates to power flattening the powerdensity distribution in a seed-blanket reactor core. To demonstrate theeffectiveness of the invention, four cases will be analyzed.

CASE 1 In this case, illustrated in FIG. I, the t2 fuel modulesconvprising cure I are all identical, i.e., each of the fuel modules I4is the same size and shape and fueled with elements of the sameenrichment. The l2 fuel modules comprising core 10 in FIG. I would,therefore, be identical to the fuel module illustrated in FIGS. 5 and5a. As previously mentioned, the gross power density distribution overmost of a reactor core of identical fuel modules is a I, shape at thebeginning of core life. The disadvantages of a .l power densitydistribution were discussed above. FIG. 1 therefore represents a priorart core for a seed-blanket reactor wherein there is no provision forflattening the power density distribution.

CASE 2 This is the subject invention and it is illustrated in FIG. 2.The geometry of annular seed region 30 in FIG. 1 in stationary assembly24 is altered in each of the fuel modules 14 around the periphery ofcore to allow additional seed rods to be loaded into the reactor core.The peripheral fuel modules are those which have sides exposed toreflector blanket l2. Mechanically, the alteration of seed region 30 inFIG. 1 in each of the peripheral fuel modules involves removing blanketrods in blanket region 32 along each side of peripheral fuel module 14that is exposed to reflector blanket 12. The structural channel (notshown) separating seed region 30 and blanket region 32 is then movedtoward decreased blanket region 32' in FIG. 2 and additional seed rodsare loaded into the region formed by the vacated blanket rods therebyforming additional seed region 29 illustrated in FIG. 2. The dottedlines in the fuel module in FIG. 2 indicate the prior boundary of seedregion 30. Prior seed region 30 and additional seed region 29 combine toform seed region 30' in FIG. 2. Each of the peripheral modules 14' inregion III has additional seed added to the three sides exposed toreflector blanket 12. Similarly, each of the peripheral fuel modules inregion II has additional seed added to the two sides of the moduleexposed to reflector blanket 12'. As will be discussed in more detailbelow, the insertion of additional seed according to this inventionresults in a flattening of the power density distribution of Case I.

CASE 3 In this case, illustrated in FIG. 3, a ring of uranium-233 oxidebearing blanket fuel rods is placed around the peripheral fuel modulesof core 10" to form additional blanket region 31. Similar to previouslymentioned blanket region 32 in core 10 (see FIG. 1), blanket region 32"is comprised of fuel rods containing a mixture of uranium-233 oxide andthoria wherein the uranium-233 oxide comprises approximately 0.5-4.0weight percent of the mixture. Additional blanket region 31, however, iscomprised of fuel rods containing a mixture of uranium-233 oxide andthoria wherein the uranium-233 oxide comprises approximately l.5 weightpercent of the mixture. Mechanically, power flattening is accomplishedby replacing the blanket rods in reflector blanket 12" around theperipheral fuel modules which contain thoria with blanket rodscontaining the above-mentioned mixture of uranium-233 oxide and thoria.This approach to power flattening the power density distribution in aseed-blanket reactor is an extension of the conventional method of fuelzoning to achieve power flattening.

CASE 4 This case, illustrated in FIG. 4, is a combination of thepower-flattening techniques described in Cases 2 and 3 providingadditional seed in the annular seed region and providing an enrichedblanket around the periphery of the core. The power flattening achievedin Case 4 will be discussed below.

It should be appreciated that the power-flattening techniques describedabove in (lanes 2, 3, and 4 can he carried out by initially selectivelyloading fuel into the various regions of the reactor core rather thanaltering an existing core as described above. As previously mentioned,the reason for describing the invention in terms of altering an existingcore is to better demonstrate the effectiveness of the subjectinvention. Table I below describes the results of power flattening inthe above-discussed cases. The core/cell factor is used to measure thedegree of power flattening in the core. This factor is the ratio oflocal power density in a fuel module in the core to the power density ina core in which all modules operate at the same power density. When thecore/cell factor is minimized and equalized in the various modules inthe core, the power density distribution is considered to be flat. Thesuperscripts I, II, and III in Table I refer to the geometrical locationof the fuel modules in regions I, II, and III of the core as discussedabove.

TABLE I.RESULTS ()l. IOWER-FLATTENING STUDIES FOR A Iii-MODULE BREEDERDEMONSTRATION (FORE Table I indicates that a l2-module breeder reactorcore with identical fuel modules (Case I) and therefore no powerflattening has a core/cell factor of l.82 in the region I fuel modules.According to the subject invention (Case 20), by adding 1.2 inches ofadditional fuel (27.l kg. U in the seed region of the peripheral fuelmodules, the core/cell factor in region l fuel modules was reduced to1.50. Furthermore, the core/cell factors in regions I] and III werechanged from I38 and 0.99 respectively in the nonpower-flattened core to1.25 and l.00 respectively by the subject invention. The averagecore/cell factor for Case I is l.37 while the average core/cell factorresulting from the invention is 1.25. As previously mentioned, when thecore/cellfactor is minimized and equalized in the various fuel modulesof the core, the power density distribution is considered to be flat.The results obtained by the method of the subject invention are moredramatically demonstrated in Case 2b where [.8 inches of additional fuel(4l.2 kg. U is added to the peripheral fuel modules. The resultantaverage core/cell factor is l.l3. The core/cell factor could be loweredmuch more by adding additional seed material, but the effect of thisadditional fuel on core shutdown properties might be detrimental. Theresults of power flattening by adding a blanket containing uranium-233around the periphery of the outer fuel modules (Case 3) indicate that82.8 kg. of uranium-233 must be added around the core to obtain anaverage core/cell factor of 1.23. Though this represents approximatelythe same average core/cell factor achieved in Case 2a, 55.7 kg. ofadditional uranium-233 are required to achieve the result. Thus, powerflattening according to the subject invention saves approximately 55.7kg. of uranium-233 and neutron leakage is minimized substantially. It istherefore possible to achieve maximum power flattening with a minimumexpense of uranium-233 by the subject invention. Case 4, which employedthe subject invention (Case 2) plus a small amount of radial zoning ofthe blanket fuel outside the peripheral seed modules, resulted in asignificant reduction in the core/cell factor. The peak core/cell valueof 1.15 occurs at the interface between the blanket fuel zones. Theradial neutron leakage for this core is 0.3 percent. Comparing Cases 3and 4, the peak core/cell value in Case 3, 1.42, can be reduced to 1.15in Case 4 by the addition of approximately 20 kg. of uranium-233according to the subject invention.

lt should be pointed out that the power flattening accorded to thesubject invention can be similarly employed in a core with a closelyspaced thoria fueled blanket instead of the relatively widely spaceduranium-233 fueled blanket of the preferred embodiment. Furthermore, thesubject invention can be employed in fuel modules that do not have axialfuel zoning in the seed regions, as in the preferred embodiment. Forexample, the invention could be used in conjunction with the thoriablanket stepped control module discussed in U.S. Pat. No. 3,335,060 toR. L. Diener. This invention can also be employed in cores withcylindrical seed regions instead of the hexagonal seed regions of thepreferred embodiment. This would be accomplished by increasing the seedthickness on the side near the periphery of the core relative to theside nearer the central modules. And similar to Case 4, the blanketreactivity may be altered by changing the fissile fuel content near theperiphery of the core relative to that in the center of the core.Furthermore, this invention is not limited to small seed-blanket coresas in the preferred embodiment. it can be applied to seed-blanketreactors in general, regardless of size.

The novelty of this invention lies in the deliberate building in of anonuniform reactivity distribution in the fabrication of theseed-blanket assemblies. The increased reactivity of the peripheralmodules causes the gross power distribution in the core to shift awayfrom the center of the core. Increasing the thickness of the seed nearthe periphery of the core also reduces the local power gradients acrossthese outer modules. In the power-flattened core the shutdown propertiesare nearly unifonn among the modules and are similar to the shutdownproperties of the central modules in an unflattened core. Powerflattening consistent with good breeding performance and adequate coreshutdown properties can be achieved with the invention described herein.Furthermore, this invention is much more efficient than conventionalradial fuel zoning to achieve the same degree of power flattening in aseed-blanket breeder reactor core.

While a particular embodiment of the invention has been illustrated anddescribed, modifications will become apparent to those skilled in theart, and it is intended to cover in the appended claims all suchmodifications which come within the scope of this invention.

What is claimed is:

1. In a seed-blanket breeder reactor core cooled and moderated by lightwater, surrounded by a reflector blanket and comprised of a plurality offuel modules each having a central, axially movable assembly including aseed region, and an outer stationary assembly including an annular seedregion surrounded by an annular blanket region, said fuel modulescomprised of seed fuel elements with identical enrichment and blanketfuel elements with identical enrichment, the improvement comprising theannular seed region of each of the fuel modules around the periphery ofthe core having sides facing the reflector blanket thicker than sidesfacing other fuel modules and thicker than sides of the annular seedregion of the centrally located fuel modules.

2. The improvement of claim 1 wherein the annular seed region thicknessof said peripheral fuel modules is 1.2 to 1.8 inches greater than theannular seed region thickness of the centrally located fuel modules.

3. The improvement of claim 1 further comprising a uranium-233 oxidebearing thoria blanket surrounding the peripheral fuel modules.

4. The improvement of claim 3 wherein said uranium-233 oxide bearingthoria blanket comprises 1.5 weight percent of uranium-233 oxide.

2. The improvement of claim 1 wherein the annular seed region thicknessof said peripheral fuel modules is 1.2 to 1.8 inches greater than theannular seed region thickness of the centrally located fuel modules. 3.The improvement of claim 1 further comprising a uranium-233 oxidebearing thoria blanket surrounding the peripheral fuel modules.
 4. Theimprovement of claim 3 wherein said uranium-233 oxide bearing thoriablanket comprises 1.5 weight percent of uranium-233 oxide.